[cyrosceals]

Hi. I have done some past year papers for my end of year exams but there are some qns I can't answer. Can anyone explain the process?

1) What product is formed when this molecule (http://img120.imageshack.us/my.php?image=lifi6.png) reacts with concentrated sulphuric acid followed by warming with water?

(Ans is that an -OH is formed at the bottom-most carbon and -OH is formed at the carbon which is attached to two -CH3 grp)

2)An organic compound requires two times its own vol of hydrogen for complete reduction in Pt catalyst. Which is the substance?

C2H2, C3H4,C4H8 
Ans: C2H2 and C4H8

3) Which is true for KI?

A)Weaker reducing agent that KCl.  (I thought it was as Cl- is smaller and hence more reactive?)
B)Forms ppt with aq AgCl, which do not dissolve in aq NH3. (?)
C)Gives brown fumes when heated with conc. H2SO4. ()
D)Formed when aq KOH is added to Iodine. (?)

4)What products are formed when this (http://img134.imageshack.us/my.php?image=cycliczq7.png) reacts with hot conc. acidic KMnO4?

5)What are the characteristics of this molecule? ( http://img50.imageshack.us/my.php?image=cyclic1tj2.png) 
One of the ans is insoluble in water. Why? I thought H-bonds btw Solute-solvent is stronger or equal to solvent-solvent interactions.

6)Lattice Energy of CaCo3 is -2200 KJ while LE of BaCo3 is +4.2 KJ. Explain why Ba (OH)2 [No, not CO3)is used instead of Ca (OH)2 to detect SMALL quantities of CO2.

[Borek]

Please read forum rules.

[cyrosceals]

Oppps. Sorry. But can anyone help me still? I attempted the whole paper but still these qns I do not understand. Pretty urgent as exam is 2-3 wks from now.

[Astrokel]

hey cyrosceals,

1 & 4 can be found in your notes. As borek mentioned, show some work!

2) Draw out the structural formula of each first.

3)  When a halide ion acting as a reducing agent, it has to lose electrons. The bigger the halide ion (I- is bigger than Cl-), the easier for it to lose because it has better shielding effect, therefore stronger or weaker reducing ability?

5) Yes, what about the large hydrophobic group, including the benzene ring?

6) Make a guess? Judging from the sign and value of lattice energy

[cyrosceals]

Hi Astrokel,

Thanks for your replies.

As for 1), maybe I have not learnt it (I only learnt the reaction mechanism for alkanes and alkenes), so do you mind if you tell me the topic the reaction is under so I can look it up or the reaction mechanism? (E.g. Phenols etc). I couldnt remember seeing such a reaction.

As for 2), I drew out the formulas but didnt understand what it means by "complete reduction". A little more hint please.

As for 3), opps... I forgot chlorine is a better OXIDIZING agent than I, not reducing. Ack....
But Im not sure about the other reactions and chemistry.

For 4), I know it is oxidative cleavage, and that the double bond breaks, R groups remain. My ans seems to be correct except that I know from my notes that all -H are supposed to become -OH (yes?), but the ans shows otherwise (that all -H remains attached).

5) Oh. Havent learnt that yet.

6)Ahhh. I just realized it while looking at the reaction. Is it that since BaCO3 is less soluble than CaCO3 which is more exo, chances of the ppt being formed from the reaction btw Ba(OH)2 and CO2 is more obvious (Less soluble)?

[Astrokel]

hey,

1) Dehydration of alkene

2) It means reduction to alkane.

3) Can you try to eliminate some of the choices? Let's say brown gas, what do you think of? Bromine gas? Is there even a source of Br- to start off with to obtain bromine?

4) Yes only the H on the aldehyde if it presence after oxidative cleavage

[Astrokel]

sorry, for 1 i mean hydration of alkene, my bad! As for 5, i don't understand why one is positive while the other is negative? I thought lattice energy is always positive(for lattice breakin) and always negative (lattice making)? Or is the question referring to enthalpy change of solution?

[cyrosceals]

Now is my bad Yah, its solution. Because the first part they ask you to find soln using LE, thats why got typo. So addition of O on H atoms which are ONLY on aldehyde? Never knew that lolz. My teacher just said add to everything except R group.

[Astrokel]

hey!

yes now we know is solution is easier to guess the answer right! 

well yes because only aldehyde are prone to further oxidation after the cleavage to carboxylic acid, so the -H becomes -OH.